I was living in the East Bay when Altamont happened. Saw the Stones at Oakland Colosseum just before Altamont, but shied away from the latter: just seemed like a bad idea on its face. The "Summer of Love" was already over and the atmosphere on the street had soured and turned pretty ugly, weed and psychedelics having given way to speed and heroin. So I guessed (correctly, as it turned out) that a "free concert" about then was probably not going to end well.
The Oakland HAs used to hang out at a pizza joint called "Pepe's"on Telegraph Avenue at that time. There was often a row of Harleys parked in front and a few patched members milling around on the sidsewalk. I'd always always cross the street and pass it on the other side because once a group of those guys got together, they tended to be volitile and totally unpredictable. It was generally prudent to just steer clear of them all together.